[The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes.]
The invention relates in general to inertial switches and in particular to very small electro-mechanical inertial switches.
To assure safety in the transportation, handling, and deployment of gun-fired and other explosive munitions, munition-fuze safety standards require that two unique and independent aspects of the launch environment must be detected in the weapon fuze system before the weapon can be enabled to arm. Examples of the aspects of the launch environment that are sensed electronically or mechanically are: setback acceleration, spin, tube exit, and airflow. Munition fuzes also perform targeting functions, which can include electromagnetic target detection, range estimation, target impact detection, or grazing impact detection.
Many of the above sensing functions can be performed either electronically or mechanically, as several examples may illustrate. First, the velocity change due to setback acceleration during tube launch can be quantified using an accelerometer and an integrating circuit, or by using a mechanical integrator (U.S. Pat. No. 5,705,767). Second, the occurrence of setback acceleration or spin acceleration can be detected with a simple inertial switch, or with an accelerometer and a threshold detection circuit. Third, target impact or grazing impact may be detected using a crush switch, an accelerometer with a threshold circuit, or an inertial switch. The best method to use for any of these functions in a given munition application depends on characteristics of the weapon system such as limitations of size, onboard system power, desired configuration, or on factors such as affordable cost, requirements for safety, or requirements for reliability.
Of the fuzing functions listed above, the present invention can be used to perform launch setback acceleration thresholding; launch setback commencement sensing, for example, to set a xe2x80x9cT-zeroxe2x80x9d timer for the fuze circuit: launch setback characterization, for example, to verify a minimum acceleration pulse duration; launch spin-up detection, for example, locate the switch away from the spin axis and orient it to sense tangentially to respond to angular acceleration; launch spin-threshold switching, for example, locate the switch away from the spin axis and orient it to respond to centrifugal acceleration; target impact switching; omnidirectional graze switching; and impact switching.
The present invention has an advantage in fuzing applications in that, being a normally-open switch, it does not draw power until actuated. This is in contrast to sensor implementations requiring continuous excitation power to operate, for example, to drive the circuit for a capacitive-coupled accelerometer. Thus, because of its extreme miniaturization, omnidirectional sensitivity, low cost, and lack of a requirement for continuous power, the present invention has widespread applications in the fuzing and instrumentation industries. For the same reason, it has numerous industrial, medical-equipment, sports, and transportation applications as well.
To accomplish some of the functions listed above, particularly for fuzing applications, various inertial switches have heretofore been devised. Some prior art devices are described in U.S. Pat. Nos. 6,314,887; 4,916,266; 4,789,762; 4,174,666; and 3,899,649; However, these switches suffer from many disadvantages in the munition fuzing application and in many other applications, as will be delineated. The present invention avoids the disadvantages of previous omnidirectional, uniaxial, or multi-axial-sensitivity inertial switches.
A problem exists in the munition fuzing industry because the need for xe2x80x9csmarterxe2x80x9d weapons often requires additional space within the weapon for signal and guidance electronics, power management, and sensors, while the need for greater lethality or payload makes simultaneous demands on volume. One solution is in the ultra-miniaturization of existing fuze functions, particularly in the area of mechanical safety and arming. There is also a need to reduce the cost of existing weapon functions to make munition systems more affordable. This need is felt acutely in small- and medium-caliber weapons because of the large numbers needed.
Another aspect of the problem is that with current trends, the domestic precision small-parts manufacturing industry is diminishing or moving overseas, so that an alternative and economical domestic source is needed for future fuze components production. The present invention has the advantage that its manufacture draws on fabrication principles and techniques from the installed domestic infrastructure of the microelectronics industry.
The prior art impact-switch implementations, in general, involve switch configurations that are too bulky, too slow-acting, are imprecise, are too expensive to manufacture, or are difficult or unsuitable to integrate with current surface-mount (hybrid circuit) or multi-chip-module-based fuze circuit implementations. These latter implementation methods are highly desirable to accommodate the aforementioned competing demands for volume in ordnance that must contain increasingly sophisticated fuzing and guidance circuits, as well as larger warheads and payloads. The state of the art as represented by prior art patents is inadequate for applications requiring extreme miniaturization, low cost, electronic integration, and the other advantages stated. Also, current day threshold switches used in fuzing typically involve glass-metal seals or polymeric materials that naturally degrade with time and changing conditions.
In an itemization of problems with the prior art, it is apparent that prior-art switches:
Are too large for, or do not offer means for, direct integration in multi-chip-modules, surface mount circuits, or even micro-controller chips;
Are expensive, due to reasons that follow;
Involve a plurality of parts that must be assembled:
Involve a domestic precision small-parts manufacturing industry that is shrinking and moving overseas;
Involve tight clearances and dimensional tolerances that are expensive to fabricate using conventional machining operations;
Involve dissimilar materials in a way that can reduce the life of the part due to differential thermal expansion, for example, metal-to-glass or metal-to-plastic seals;
Involve polymeric parts whose material may degrade with time and thermal cycling, or whose function varies with temperature;
Do not take advantage of recent micro-scale fabrication technologies that use principles and processes well known and widely utilized in the micro-electronic fabrication industry, e.g., optical fabrication masking directly from CAD layouts, optical exposure, chemical developing and rinsing, to create three-dimensional mechanical structures;
Use materials that can corrode.
In summary, there is need for an extremely small, inexpensive, fast-acting, surface-mount- or flip-chip-integratable, tailorable, multi-output impact/torsion switch in military weapons, medical equipment, and industrial and automotive applications.
The present invention meets the need for an extremely miniature, very low cost, fast-acting, unpowered, omnidirectional impact switch. In particular, there is a need in the munitions fuzing area for an ultra-miniature, inexpensive, omnidirectional, fast-acting impact-switch, also known as a xe2x80x9cg-switchxe2x80x9d, that can be integrated with a fuze circuit. The need for small size comes from the increasing miniaturization required to pack more functionality into small caliber weapons, e.g., a 20 -mm bursting-round fuze, which also must contain sophisticated timing, sensing or targeting electronics and whose payload must be maximized for effect. This puts space inside the projectile at a premium.
The present invention can function as a xe2x80x9cT-zero xe2x80x9d switch to initiate processes within a fuze circuit or start a time-from-launch counter or some other function. It can also function as a graze or impact switch for sensing target or ground impact, for purposes such as actuating the firing circuit or starting a self-destruct delay timer or starting a fire-circuit bleed-down timer. The invention can also function as a penetration-layer counter. In addition, there are many non-munition applications for the invention. The threshold values for a particular embodiment of the invention can be set through selection or specification of the gaps or spring rate specified in the layout and assembly drawings. The invention meets the need for an extremely small, surface-mountable, inexpensive omnidirectional impact switch.
The invention has differences that give beneficial results that are not cited in earlier art. These differences are important because:
The invention allows an extreme degree of miniaturization relative to prior-art impact switch implementations.
The invention allows for efficient methods of electrical connection to, and integration with, a circuit. For example, in a fuze application, the invention can be integrated directly with the fuze controller circuit via surface-mount techniques on a hybrid circuit board, or it can be flip-chip integrated directly with fuze ASIC chips, and may in time be possible to integrate on the same substrate with or as part of an ASIC or microcontroller chip itself.
The invention allows for low cost of manufacture by using technology related to the semiconductor wafer and silicon chip manufacturing industry. By virtue of wafer-to-wafer bonding techniques in a semiconductor foundry clean-room environment, and subsequent dicing, the following advantages are obtained:
Simultaneous assembly of hundreds, or thousands, of devices by the joining of two substrate wafers in a wafer-to-wafer bonding process. Wafer-to-wafer bonding in effect accomplishes assembly and electronic packaging in one operation.
Hermetic sealing of the devices.
Extreme cleanliness due to fabrication in a clean-room environment.
Amenability to automated inspection and testing.
Avoidance of the prior art operations of piece-parts manufacture (industrial stamping, drawing, crimping, bonding, welding, sealing, etc.), sorting, handling, and pick-and-place assembly.
The invention can be realized with a variety of sensitivity thresholds depending on specific design factors such as contact gaps, spring stiffness, magnitude of the proof mass, or contact-electrode geometry. The invention allows for different sensitivity levels in different axes, by making modifications to the contact electrodes. The invention also can be configured to provide sensitivity to torsional inputs, and can indicate direction of inputs. Also, the configuration and connection of contacts can allow for sensitivity in only one axis or one direction.
In general, the invention is a normally-open, momentary, non-latching, inertial thresholding switch, fabricated on a substrate in a planar configuration, using no cylindrical tilt mass, with low mass and small switch gap to allow fast switch action and rapid reset. Of rugged construction, its high mechanical frequency limits sensitivity to vibration inputs.
The invention may be extremely small (about 1 cubic mm), integratable with electronics, surface-mountable, rugged, cheap and fast-acting. The invention does not draw power, has a large dynamic range, has different sensitivity in different axes, and can be ganged with identical sensors or an array of sensors with different thresholds on the same substrate. The invention offers a number of improvements over the prior art:
It incorporates a monolithic, micro-machined, integral mass-and-spring, fabricated and integrated simultaneously on a planar substrate with its electrical contacts, assembled with a one-piece cover plate in a wafer-scale assembly process, individuated from the whole by dicing. This is a fundamentally different approach for a miniature g-switch than has been done before.
It has a suspension spring formed in lithographic or lithographic-derived process, integral with the proof mass.
It has a centrally-located mechanical anchor and peripheral proof mass (unique configuration).
It is simple, i.e., a MEMS device on a substrate plus a cover plate.
It is free from the cylindrical tilting-mass approach of prior designs.
Its actuation mechanism does not require xe2x80x9ctiltingxe2x80x9d of a cylindrical mass, with the associated moments of inertia, hence, it can act faster.
The switching gap can be extremely small, leading to fast switching action of less than 200 microseconds.
The approximate size of the assembled embodiments is 1 -mmxc3x971 -mm on substrate with a thickness of 500 to 1000 microns, for a total volume of 1 to 5 cubic mm, compared with 90-500 cubic mm for prior art switches.
Approximately 2,000 to 10,000 devices can be fabricated on an 8xe2x80x3 wafer substrate.
As a MEMS-fabricated device, it is its own xe2x80x9chousings xe2x80x9d and hermetic seal.
The configuration of a center-supported spring/mass assembly optimizes the size of proof mass relative to overall device footprint and also makes it easy to run contact leads around the outside of the mass and simplifies fabrication (no need to deposit tracks and then insulate them from the mass).
The invention will be better understood, and further objects, features, and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings.